This disclosure relates generally to a method and an arrangement for separating CO2 from combustion exhaust gas. More particularly, this disclosure relates to a method and an arrangement for separating CO2 from combustion exhaust gas that originates in a furnace through the combustion of carbon based fuels or wastes with the aid of air as an oxidizing agent.
Such a method has become known from the publication “Process Optimization in Postcombustion CO2 Capture by means of Repowering and Reversible Carbonation/Calcination Cycle” by Luis M. Romeo et al., ENDESA, Spain, 8th Int. Conf. on Greenhouse Gas Control Technologies, June 2006, Trondheim, Norway.
In this known method, a large part of the CO2 contained in the combustion exhaust gas is separated by means of a carbonate looping system or reversible carbonation/calcination cycle that is arranged downstream of the furnace (postcombustion) as seen in the direction of flow of the combustion exhaust gas, and is sent for further treatment. To separate the CO2, the sorbent CaO, i.e., calcium oxide or unhydrated lime, is used and is carried in the loop inside the carbonate looping system. The CaO, which is produced or regenerated in the calciner at a temperature of 900-950° C., is sent at this temperature to the carbonator through a duct that connects the calciner with the carbonator. In the carbonator, into which the combustion exhaust gas containing the CO2 is directed from the furnace and which has a circulating fluidized bed, at a temperature of approximately 650° C. the CaO absorbs a large part of the CO2 contained in the combustion exhaust gas to the calcium carbonate CaCO3. The solids/gas mixture leaving the carbonator is sent to a separator in which the solids are separated from the combustion exhaust gas. The reduced-CO2 combustion exhaust gas is sent into the atmosphere by means of a treatment facility that may be present, and the solids, essentially CaCO3 but also including ash, CaSO4 and CaO, are sent through ducts to the calciner. In the calciner, CaCO3 is heated or calcinated at a calcination temperature of 900-950° C., thereby creating one solid and one gaseous product. The solid product essentially exhibits CaO regenerated from the CaCO3 and the gaseous product essentially exhibits CO2 and water. Since calcination takes place as an endothermic reaction, heat must be brought into the calciner in order to furnish the required calcination or reaction temperature. This takes place by means of an additional firing in the calciner, whereby coal is burned by means of the oxidizing agent or oxidant O2 and heat is thus brought into the calciner, which is equipped with a circulating fluidized bed. In order to avoid diluting the gaseous product that originates in the calciner, e.g., with nitrogen, thus making the separation of CO2 more difficult, O2 is mandatorily used in place of air as the oxidant for the additional firing. The regenerated CaO is sent from the calciner to the carbonator as the sorbent with a product heat of approximately 900° C. With the same product heat, the gas product, which essentially contains CO2, is diverted from the calciner and the CO2 is sent for further treatment, and in this case cooled, compressed and transported for storage. The method described above runs continuously.
What has proven to be disadvantageous about the known method or arrangement is the fact that this method or arrangement is afflicted with a loss of efficiency of approximately 3 percentage points. This loss of efficiency can be essentially attributed to the calciner, which is equipped with additional firing and is thus fired, during which technically pure oxygen is used as the oxidant or oxidizing agent for burning the fuel, i.e., coal. In that regard, the production of oxygen for use as the oxidant in the calciner is associated with a high expenditure of energy that leads to the loss of approximately 3 efficiency percentage points.
The task of the invention is now to create a method for separating CO2 from combustion exhaust gas that originates in a furnace through the combustion of carbon based fuels or wastes with the aid of air as an oxidizing agent that avoids the previously mentioned disadvantages. In particular, it is the task of the invention create a method for separating CO2 from combustion exhaust gas that originates in a furnace through the combustion of carbon based fuels or wastes with the aid of air as an oxidizing agent, in which the need for additional firing, and thus the need for oxygen for the calciner, is reduced or can be eliminated entirely. It is also a task of the invention to suggest an arrangement for separating CO2 from combustion exhaust gas that originates in a furnace through the combustion of carbon based fuels or wastes with the aid of air as an oxidizing agent.